EP0760076B2 - Firing furnace with a revolving firing zone and a cooled internal wall - Google Patents

Description

TECHNICAL AREA

The invention relates to a rotary kiln for cooking carbonaceous products and in particular carbon anodes used for the production of aluminum by electrolysis according to the Hall-Héroult process.

STATE OF THE ART

Rotating fire furnaces, such as that described in the European patent application EP-A-0 541 165, generally comprise two parallel rows of juxtaposed rectangular-shaped chambers in which anodes or other green carbonaceous products are fired. These rooms, separated by transverse walls, and whose bottom and side walls are thermally insulated by an insulating brickwork, are subdivided into several compartments or cells containing the products to be cooked. These cells are separated by thin-walled hollow partitions arranged parallel to the major axis of the furnace. In these hollow partitions circulate hot gases, whose temperature can reach 1150 or 1200 ° C. They are formed essentially by the combustion gases generated by one or more burner ramps that are moved from one chamber to the next as the cooking takes place.

Rotating furnaces, open-type furnaces and closed-type furnaces are often distinguished. The difference lies mainly in the constructive characteristics of the interior brickwork leading to various paths of the hot gases circulating around the cells containing the loads to be cooked, but the general organization of these rotary kilns remains the same with the existence of 2 rows parallel rooms, whose length can reach nearly 200 meters, connected at their ends by flues.

• soit un cuvelage unique renfermant les 2 rangées de chambres et leurs isolations; configuration souvent adoptée pour les fours à chambres de type fermé;
• soit un cuvelage double, chaque cuvelage renfermant une rangée de chambres; configuration souvent adoptée pour les fours à chambres de type ouvert.

These rows of parallel chambers thermally insulated on 3 sides (lateral insulation + bottom) are contained in a concrete casing formed of a horizontal slab or bottom of tank and 2 walls or lateral vertical sails. At this stage, two modes of construction are possible with their respective advantages and disadvantages by implementing:

• a single casing containing the 2 rows of chambers and their insulation; configuration often adopted for closed chamber furnaces;
• a double casing, each casing enclosing a row of chambers; configuration often adopted for open type ovens.

This is a significant constructive difference but not related to the type of room ovens; in other words, a double-cased closed type furnace is entirely feasible as is a single casing open type furnace.

The advantage of the unique casing is its great compactness and therefore the resulting saving of space as well as the savings it entails in terms of investment costs, but this to the detriment of good performance and therefore longevity insulating briquetting which is arranged between the 2 rows of rooms. Indeed, these briquetting bear significant thermal and mechanical stresses due to the amplitude of the temperature variations to which they are subjected and thrusts exerted by the chambers located on either side of these briquetting. Due to the difficulties of ensuring effective cooling of this central zone, the cooling of the single casing is limited in most single-cased industrial furnaces to a simple natural ventilation of the walls or outer walls of the casing. This differential heat treatment of the central zone with respect to the outer zones also generates mechanical and thermal stresses created by the high thermal gradient existing between the chambers facing each other in the two rows of chambers constituting the furnace.

The major advantage of the double bowl is that the load constituted by the row of chambers, is treated symmetrically with respect to its longitudinal axis. In particular the inner sails are cooled like the outer sails in the most frequent case where there is a natural ventilation provided in particular by the circulation channels or vaults which cool the bottom of the oven. This configuration therefore makes it possible to extend the lifetime of the briquetting, with, however, the disadvantages of a higher realization cost and a larger footprint with identical production capacity.

PROBLEM

The production of a chamber oven for cooking carbon products simultaneously presenting the advantages of single-cased and double-cased furnaces, namely: on the one hand a limited space requirement and a moderate production cost, on the other hand a large longevity of brickwork including central briquetting, remains a problem poorly resolved for the builder or the user of a rotating furnace.

OBJECT OF THE INVENTION

The invention provides a solution to this problem. It relates more specifically to a rotary fired furnace for cooking carbon products including in particular 2 parallel rows of cooking chambers. placed with their own thermal insulation jointly or separately in a casing, preferably made of concrete, at least cooled by ventilation of its external lateral walls or external walls, characterized in that the 2 parallel rows of cooking chambers are separated by an internal veil which is specifically cooled and made integral with the bottom of the casing or write off.

This internal web, the thickness of which varies in particular according to the nature of the materials used for its construction, comprises a network of vertical, horizontal ducts or any other geometry allowing the circulation of a coolant liquid or gaseous, allowing lowering the temperature of the constituent material (s) of the central web to an acceptable value by said materials. Preferably the inner web is formed by 2 vertical uprights of identical thickness, symmetrical with respect to the vertical axis of symmetry, linked to their lower ends at the bottom of the casing or raft. These amounts are maintained at fixed spacing by vertical or horizontal spacers so as to create vertical or horizontal corridors in which circulates the cooling fluid, which is preferably ventilation air supplied by appropriate ventilation means. In general, the spacers are made of the same material as the vertical uprights.

Suitable means of ventilation include natural ventilation, forced ventilation or mixed ventilation. These ventilation means are grouped together with the other auxiliary means essential for driving the oven, including control and regulation means and lifting and handling means, in the building that houses the oven.

DESCRIPTION

• . Fig.1 représentation schématique en vue de dessus d'un four à feu tournant à cuvelage double selon l'art antérieur.
• . Fig.2 représentation schématique en coupe transversale selon B,B' du four à cuvelage double de la fig.1 selon l'art antérieur.
• . Fig.3 représentation schématique en coupe transversale d'un four à cuvelage unique selon l'art antérieur.
• . Fig.4 représentation schématique en coupe transversale d'un four à feu tournant avec un voile interne ventilé selon l'invention.
• . Fig.5 représentation en perspective coupée du voile interne ventilé.

The invention will be better understood from the following detailed description illustrated by the following figures:

• . Fig.1 schematic representation in plan view of a rotating furnace with double casing according to the prior art.
• . Fig.2 schematic cross-sectional representation according to B, B 'of the double casing furnace of Fig.1 according to the prior art.
• . Fig.3 schematic cross-sectional representation of a single casing furnace according to the prior art.
• . Fig.4 schematic cross-sectional representation of a rotating furnace with a ventilated internal web according to the invention.
• . Fig.5 cutaway perspective representation of the ventilated internal web.

FIG. 1 is representative of a rotary firing furnace 1 comprising two parallel rows 2a and 2b of juxtaposed chambers 3 of rectangular shape separated by transverse walls 4 and subdivided into cells 5 containing the product to be fired. These cells are separated by hollow partitions 6 with thin walls parallel to the major axis A, A 'of the furnace in which circulate the gases for heating the charges (not shown) placed in the cells. These hot gases are mainly combustion gases generated by one or more ramps 7 of burners that are moved from one chamber to the next as the cooking of the product takes place.

According to Figure 2 the 2 concrete casings 8a and 8b respectively contain the chambers 3a and 3b with their thermal insulation 17a and 17b. The external sails 9a and 9b as well as the 2 inner sails 10a and 10b of the two casings are cooled by natural ventilation as well as the 2 rafts 12a and 12b equipped with vaults 11a and 11b. The distance between the 2 outer faces of the inner sails is of the order of 2 to 3 meters to allow sufficient ventilation of the two casings by natural draft.

According to FIG. 3, the two parallel rows 2a and 2b of chambers 3a, 3b are contained with their respective thermal insulation 17a and 17b in a single casing 8 of concrete formed by 2 outer vertical sails 9a, 9b resting on a concrete base 12 . In this configuration the frequency of maintenance interventions on the central brickwork is much higher than that observed for brickwork of rooms having their individual casing which, independently of the direct cost of higher maintenance, can lead to a limitation of the capacity. of production.

Note however that with equal production capacity, the width reduction of about 3 meters of a single casing kiln compared to a double casing furnace allows a reduction in floor area of 300 to 600 m ² according to the length of the oven.

According to FIGS. 4 and 5, representative of a ventilated central fan rotating firing furnace of the invention, there are interposed between the thermal insulators 17a and 17b of the chambers 3a and 3b, and more precisely between the central briquetting, a vertical central veil. or inner sail 10, preferably made of concrete, the total thickness of which varies according to the capacity of the furnace and more precisely the importance of the thermal flows to be evacuated, but also the nature of the material used and in particular its thermomechanical properties , crush and thermal shock resistance, thermal conductivity ... This web 10 has a total thickness in its central and upper parts of between 200 and 900 mm and preferably between 400 and 600 mm. It generally widens gradually to its base resting on the casing floor or base 12. The veil 10, made mostly of concrete identical to that of the casing 8, is preferably formed by 2 uprights vertical 13 and 13b of identical thickness, symmetrical with respect to the vertical axis CC '. These uprights 13a, 13b secured to their lower ends 14a, 14b of the slab 12, are kept at constant spacing "e" by spacers preferably vertical or horizontal 15 of rectangular section of 2 to 10 dm ² placed at a regular spacing between the 2 amounts to limit the deformation, but also to form vertical or horizontal corridors 16 communicating at their ends with unrepresented ventilation means, said ventilation being natural, forced or mixed.

The spacing between two partitions defined by the vertical or horizontal spacers 15 is between 50 and 500 mm, but preferably between 100 and 300 mm, so as to provide sufficient ventilation in a minimum volume. As for the spacing of the spacers 15, it is determined according to the deformation stresses to which said amounts are subjected under the effect of particular charges and temperature. This spacing is most often between 0.5 and 1 meter.

According to this new configuration, the inner sail 10 cooled and limited thickness therefore does not lead, all things being equal, significant expansion of the furnace rotating, as is the case with the double-casing furnace. On the other hand, being cooled naturally, or forcibly by ventilation of air, or by any other cooling fluid, it makes it possible to recreate the mechanical and thermal symmetry that is lacking in the single vessel.

The choice of the ventilation mode depends mainly on the importance of the heat flow to be removed from the central zone of the oven to limit the temperature peaks in the brickwork. For a large capacity oven, it will be advantageous to use a forced ventilation unit powerful enough to inject fresh air, preferably from the bottom to the top of the vertical corridors 16 of the inner veil 10, possibly by the intermediate of the vaults 11 of the raft, and this while maintaining for the outer sails 9a, 9b, a natural ventilation.

Forced ventilation can also be performed by injecting fresh air into the horizontal corridors 16, that is to say longitudinally from one end of the inner web 10 to the other end. The injection of fresh air by blowing or aspiration is carried out using appropriate mechanical means, such as fans or extractors, located at one end of the furnace.

EXAMPLES OF APPLICATION

For the cumulative advantages previously enumerated, this new concept of rotary kiln obviously applies to the construction of new furnaces whether open or closed, but it also finds interesting applications in the transformation of existing furnaces which are suitable for improve performance during a renovation.

Thus, in the case of an open double casing furnace to be renovated, it is easy, with constant floor space, to connect the two rafts 12a, 12b and bring together the two internal walls 10a and 10b which then act as uprights 13a and 13b simply by maintaining a spacing sufficient to form an air flow chimney 16 communicating at its lower part with the existing vaults 11. It is thus possible to increase the width of the 2 rows of chambers by 1 to 2 meters and consequently to increase the volume of the cooking load of 10 to 20%.

Similarly, in the case of a single closed casing oven that is to be converted into an open oven for example to increase the capacity. It is possible to achieve this objective without increasing the footprint by applying the concept of the invention which is here to interpose an internal ventilated web 10 from 400 to 600 mm thick, connected to the base 12 of the invention. Since the central veil occupies only a very limited part of the useful volume provided by the closed oven casing, the production capacity provided by the open oven is fully benefited by its better load / cook volume ratio. bricklaying and this at constant ground congestion.

Claims (10)

1. Baking ring furnace (1) used to bake carbonaceous compounds made notably of two parallel rows (2a, 2b) of baking chambers (3a, 3b) placed with their own thermal insulation (17a, 17b) jointly or separately in a basin 8, usually made of cement, cooled at least by ventilation of its two exterior walls or partitions (9a, 9b) characterized by the fact that the two parallel rows (2a, 2b)of baking chambers (3a, 3b) are separated by a unique interior partition (10) which is specifically cooled and made firmly attached to the bottom of the basin (12), and that the cooled interior partition (10) comprises a network of vertical, horizontal or any other geometric shape ducts or passages (16) allowing the circulation of a cooling liquid or gaseous fluid.
2. Baking ring furnace according to claim 1 characterized by the fact that the interior partition (10) is made of two vertical members (13a, 13b) of identical thickness, symmetrical compared to the vertical axis CC', attached at their lower ends (14a, 14b) to the bottom of the basin (12) and maintained at a fixed spacing "e" by vertical or horizontal spacers (15) placed at fixed intervals in order to create vertical or horizontal passages (16).
3. Baking ring furnace according to claim 2 characterized by the fact that the interior partition (10) has a total width in its central and higher parts comprising between 200 and 900 mm and preferably 400 and 600 mm.
4. Baking ring furnace according to claim 2 characterized by the fact that the spacing "e" between the two members (13a, 13b) is 50 mm <e<500 mm and preferably 100 mm ≤ e ≤300 mm.
5. Baking ring furnace according to claim 2 characterized by the fact that the regular space between the spacers (15) is between 0.5 and 1 meter.
6. Baking ring furnace according to claim 2 characterized by the fact that the members (13a, 13b) are made of the same cement as the basin (8).
7. Baking ring furnace according to any of claims 1 to 6 characterized by the fact that the cooling of the interior partition (10) is insured by circulation of air in the passages (16) communicating at their ends with natural ventilation means, forced or mix.
8. Baking ring furnace according to claims 1 or 7 characterized by the fact that the forced ventilation is realized by injection of cool air, from the bottom to the top of the vertical passages (16) of the interior partition (10), optionally using the ventilation ducts (11) of the basin whereas the exterior walls (9a, 9b) are cooled by natural ventilation.
9. Baking ring furnace according to claims 1 or 7 characterized by the fact that the forced ventilation is realized by injection of cool air, longitudinally from one end of the interior partition (10) to the other, in the horizontal passages (16).
10. Baking ring furnace according to claim 9, characterized by the fact that the forced ventilation is done by blowing or aspiration of cool air using appropriate mechanical means situated at one end of the furnace.

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